1. Field of the Invention
This invention relates to an image forming apparatus such as a copier or a printer, and more particularly pertains to a fixing unit for use in such an image forming apparatus for fixing a toner image onto a copy sheet. The fixing apparatus incorporates a heating roller or heat roller for fusing a toner image on the copy sheet and a pressing or pressure roller which is rendered in pressing contact with the heating roller to form a nip through which the copy sheet is transported for image fixation. This invention also relates to such an image forming apparatus provided with the fixing unit.
2. Description of the Related Art
A heater-type fixing unit for use in an image forming apparatus such as a copier and a printer generally has a cylindrical heating roller heated to a certain temperature and a cylindrical pressing roller which is rendered in pressing contact with the heating roller to form a nip between the heating and pressing rollers. A toner image which has been transferred onto a copy sheet, is fused by the heating roller and fixed on the copy sheet while the copy sheet is passing through the nip defined by the heating roller and the pressing roller.
Generally, in the above-mentioned fixing unit, the pressing roller is composed of a resilient material and is pressed against the heating roller. Specifically, the nip is defined by the contact surface on the outer surface of the heating roller and the contact surface of the pressing roller which is in pressing contact with the heating roller. While a copy sheet bearing a toner image is transported through the nip defined by the heating roller and pressing roller which are rotating, the toner image is fixed on the copy sheet by the heat of the heating roller and the pressing force is exerted to the copy sheet by the pressing roller.
Once the fixing unit is initialized for operation, the pressing roller of resilient material is constantly in pressing-contact with the heating roller at the nip with the result that it is highly likely that the resilient property of the pressing roller is deteriorated as time lapses in the case where the fixing unit is left unused for a long term, because the pressing roller is always under stress.
In view of the above, it is often the case that the image forming apparatus equipped with the fixing unit of the above mentioned construction is shipped or delivered to a customer with the pressing roller being kept off or away from the heating roller, namely, in a released state where the pressing roller is released from the pressing-contact against the heating roller. The pressing-contact is released to prevent the pressing roller having a certain resilient property, from being deformed due to unnecessary application of external force during a time period e.g. from shipment to installment and initialization of the image forming apparatus for actual use. In such a condition, after purchasing the image forming apparatus, a user has to resume the fixing unit to a pressing-contact state by him/herself to render the image forming apparatus (fixing unit) to an operable state.
An arrangement is conventionally known in which the fixing unit is selectively brought to a pressing-contact state where the pressing roller is pressed against the heating roller and a released state where the pressing roller is kept away from the heating roller by the manipulation of a control lever associated with the fixing unit. However, even with provision of such a control lever, a user may forget to set the control lever to the pressing-contact state, or inadvertently attempt to activate the image forming apparatus without setting the fixing unit to the pressing-contact state. With such a user""s inadvertent operation and/or negligence of resuming the fixing unit to the pressing-contact state, it is highly likely to occur a mal-function of the image forming apparatus.
For example, the heating roller may be overheated as the pressing roller is not in contact with the heating roller and the heat produced by a heater within the heating roller is not shared by the heating and pressing rollers so that the heat roller may be heated rapidly above an allowed level before thermo-control system reacts, resulting in damage of the heat roller in worst case. A copy sheet may be jammed in the gap between the heating and pressing rollers in disengaged state, thus leading to failure in discharging the sheet. The image forming apparatus may indicate that an error is occurring and stop its operation, in response to detection of mal-function of the apparatus, such as the over-heating of the heating roller or sheet delivery error.
In view of the above, there has been proposed a fixing unit 101 as shown in FIG. 8.
The fixing unit 101 shown in FIG. 8 incorporates a heating roller 103 and a pressing roller 102. Control levers 105 are provided in the fixing unit 101 to selectively set the fixing unit 101 between a pressing-contact state and a released state. At the time of shipment or delivery, a strip member 104 as shown in FIG. 8 is adhered by a manufacturer to each control lever 105 with a front surface of the strip bearing a message to indicate a user the necessity of setting the lever to the pressing contact position. The message indicates a user that the control levers 105 should be manipulated in such a direction as to render the pressing roller 102 in pressing contact against the heating roller 103 before the user activates the image forming apparatus.
Specifically, each strip member 104 is adhesively attached to the front surface of the control lever 105 in such a manner that a user can notice the message on the front surface of the control lever 105 while the control lever 105 of the fixing unit 101 extends downward from the pressing roller 102. The pressing roller 102 is kept off or away from the heating roller 103 when the control lever 105 extends downward and the annular portion 106a of a cam 106 engages the disk 107 which is provided on opposite ends of the heating roller 103 coaxially with but independently of the heating roller 103. The cams 106a is provided on opposite ends of the pressing roller 102 coaxially with but independently of the pressing roller 102. The strip member 104 bears, for example, a message on the front surface thereof to indicate the a user that the fixing unit 101 should be brought to a pressing-contact state by turning the control lever 105 upward before use, until the recessed portion 106b of the cam 106 comes into engagement with the disk 107.
Although not illustrated in FIG. 8, the message may be any message as far as the message indicates or warns a user to use the apparatus under a requested condition, for example, xe2x80x9cLift the lever up before use!xe2x80x9d The strip member or seal 104 is usually adhered to the free end of the control lever 105 sideway as shown in FIG. 8 to facilitate removal of the strip 104 before initiation of the image forming apparatus.
However, there sometimes arises a case that a user may inadvertently remove the strip member 104 without reading the message thereon and attempt to activates the apparatus, leaving the control lever 105 at the initial downward extending state to keep the heating roller 103 away from the pressing roller 102 despite the fact that the message is displayed on the strip member 104 on the front surface of the control lever 105, i.e., a site where the user can notice the message. Thus, the above idea of attaching the strip member 104 to urge the user to use the apparatus in a certain requested condition may fail because the user may forget to manipulate the control lever 105 according to the instruction on the strip member 104, for instance. In such a case, as described above, an error indication may occur during activation of the apparatus, the heating roller may be damaged or broken due to the overheat when the apparatus is activated, or a copy sheet may be jammed.
Also, the conventional fixing device 101 may be constructed such that the control lever 105 is set to an intermediate position indicated by the symbol {circle around (3)} (see FIG. 9) where a heating roller 103 and a pressing roller 102 are rendered in light or slight contact with each other or kept away from each other with a little clearance so that a user can easily remove a jammed sheet. Hereinafter, the position {circle around (3)} is called xe2x80x9cmaintenance positionxe2x80x9d as the removal of the jammed sheet is considered as operation for maintaining the image forming apparatus.
Referring to FIG. 9, the fixing unit 101 is provided with a covering member 106 covering the heating roller 103 and pressing roller 102, and is installed in a housing 3 of an image forming apparatus. The covering member 106 is formed with guide slots 9 through which the control levers 105 extend such that the control levers 105 are guided by the slots 9. A door 31 is hinged on sidewalls of the housing 3 to open and close the opening of the housing 3 for the access to the control levers 105. When a user opens the door 31 and sets the control lever 105 to the maintenance position {circle around (3)}, the user is allowed to access the interior of the housing 3, and to pull out the jammed paper at upstream or downstream of the fixing unit not shown but in a conventional way.
If the user attempts to close the door 31 with the control lever 105 being left at the maintenance position {circle around (3)}, for instance, then, the control lever 105 left at the maintenance position {circle around (3)} blocks the door 31 from being closed, and the user is reminded of returning the control lever 105 from the maintenance position {circle around (3)} to an operable position {circle around (2)} corresponding to a pressing-contact state of the fixing unit 101 where the pressing roller 102 is pressed against the heating roller 103. However, if the user inadvertently shifts the control lever 105 to an initial position {circle around (1)} corresponding to a released state of the fixing unit 101 where the pressing roller 102 is kept away from the heating roller 103, even in such a condition, the user is enabled to close the door 31 without interference of the control lever 105. Thus, this arrangement fails to prevent erroneous operation of a user of manipulating the control lever 105 from the maintenance position {circle around (3)} to the initial position {circle around (3)}.
In view of the above, it is an object of this invention to provide a fixing unit for an image forming apparatus constructed to avoid operator""s failure in setting the fixing unit to a normal-use condition.
It is another object of the present invention to provide a fixing device for an image forming apparatus capable of avoiding activation of the fixing device under inadequate condition.
It is still another object of the present invention to provide a fixing device for an image forming apparatus, having a safety or fail-safe mechanism for leading a user to set the fixing device to a normal operating condition before activating the fixing unit.
It is a further object of the present invention to provide a fixing device for an image forming apparatus, in which a mechanism prevents a user from inadvertently setting the fixing device to an unfavorable condition. It is still further object of the present invention to provide an image forming apparatus including a fixture attaining one or more of the objects as mentioned above.
According to an aspect of this invention, a fixing unit according to this invention comprises: a heating roller for fusing a toner image on a sheet; a pressing roller which is rendered in pressing contact with the heating roller to form a nip between the heating roller and the pressing roller with the sheet being transported through the nip; a control member selectively settable to a first position for keeping the pressing roller off the heating roller and to a second position for keeping the pressing roller in press-contact with the heating roller; and a manipulation member operable to shift the control member between the first and second positions, and an indication member for indicating necessity of shifting the control member from the first to second position before the actuation of the fixing unit, the indication member is associated with the manipulation member such that removal of the manipulation member is accompanied by the shifting of the control member from the first to second position.
According to an embodiment of the present invention, the control member is in the form of a control lever which pivots to selectively set the fixing unit to a pressing-contact state where the pressing roller is pressed against the heating roller and a released state where the pressing roller is off the heating roller. The indication member has a form of a sheet which is detachably attached to the control lever in such a manner that the indication member is removed from the control lever by pulling the indication member in a direction to turn the control lever from the first to second position.
The above mentioned aspect of the present invention prevents the user from forgetting to set the fixing unit to the pressing-contact state after removal of the indication member, since the shifting of the control member from the first to second position when the user try to remove the indication member.
According to another aspect of the present invention, the fixing unit is provided with a block mechanism to prevent the user from erroneously shifting the control member from the first to second position. According another form of the embodiment of he present invention, the control lever is settable to a third position for releasing the pressing contact of the pressing roller with the heating roller and the block mechanism is constructed to prevent the control lever from moving to the first position from the second or third position.
These and other objects, features and advantages of the present invention will become more apparent upon a reading of the following detailed description and accompanying drawing.